1. Field of the Invention
The present invention relates to an electrostatic chuck used to fix a flat substrate (flat test sample), such as a semiconductor wafer, a metal wafer, or a glass plate, that is used to manufacture a liquid crystal display (LCD) and semiconductor devices, such as IC, LSI, and VLSI.
2. Description of Related Art
In the related art, in a method of manufacturing a semiconductor device or a liquid crystal display device, various processes are performed on the surface of a flat substrate, such as a semiconductor wafer, a metal wafer, or a glass plate. During these processes, in order to fix the flat substrate using an electrostatic attraction force and keep the flat substrate at a constant temperature, electrostatic chucks having various structures have been proposed (for example, Patent Document 1 (Japanese Patent No. 3176305)).
FIG. 13 is a cross-sectional view illustrating an example of an electrostatic chuck according to the related art. In an electrostatic chuck 1, a ring-shaped circumferential wall 3 having a predetermined width and height is provided in a circumferential portion of a surface 2a of an electrostatic chuck body 2 that is formed of an insulating material, and a plurality of columnar protrusions 4 having the same height as the circumferential wall 3 are provided on the surface 2a inside the circumferential wall 3. A roughening process is performed on top surfaces 4a of the columnar protrusions 4, that is, contact surfaces of the columnar protrusions 4 with a flat substrate W such that a surface roughness Ra (centerline average roughness) is in the range of 0.35 μm to 3.0 μm, and the flat substrate W is supported by the top surface 3a of the circumferential wall 3 and the top surfaces 4a of the columnar protrusions 4.
A cooling gas introduction hole 5 is formed in the electrostatic chuck body 2 in the thickness direction thereof, and an internal electrode 6 for generating an electrostatic attraction force is arranged in parallel to the surface 2a. When a voltage is applied to the internal electrode 6 through a lead electrode 8 provided in an electrode insertion hole 7, the flat substrate W is attracted to the electrostatic chuck by an electrostatic force, and the temperature of the flat substrate W is maintained at a constant value by a cooling gas flowing through the cooling gas introduction hole 5.
In recent years, a so-called waferless dry-cleaning process of cleaning the inside of a reaction chamber before a flat substrate is fixed to an electrostatic attraction surface by an electrostatic attraction force has been performed on apparatuses using plasma, such as a plasma CVD apparatus, a plasma etching apparatus, a sputtering apparatus, and an ion injection apparatus, particularly, on the plasma etching apparatus, in order to remove foreign materials or particles from the electrostatic attraction surface of an electrostatic chuck provided in the apparatus.
However, in the electrostatic chuck 1 according to the related art, as the number of flat substrates W processed increases, the top surfaces 4a of the columnar protrusions 4 of the electrostatic chuck body 2 are worn away due to contact with the flat substrate W, and the states of the top surfaces 4a of the columnar protrusions 4 vary over time, which makes it difficult to keep uniform contact between the top surfaces 4a and the flat substrate W.
In this case, thermal conductivity between the electrostatic chuck body 2 and the flat substrate W varies over time, which makes it difficult to keep the flat substrate W at a constant temperature. As a result, it is difficult to process each flat substrate W under the same condition.
In addition, particles are likely to be inserted into a gap between the top surfaces 4a of the columnar protrusions 4 and the contact surface of the flat substrate W. When the particles are inserted into the gap, the electrostatic attraction force is weakened, and the sealing of a cooling gas deteriorates.
Meanwhile, since the surface of the electrostatic chuck on which the flat substrate is loaded is directly exposed to plasma during the waterless dry-cleaning process, the state of the mounting surface varies over time as the number of waferless dry-cleaning processes increases. As a result, the contact state of the mounting surface with the flat substrate varies, and the thermal conductivity between the mounting surface of the electrostatic chuck and the flat substrate varies over time, which makes it difficult to keep the flat substrate at a constant temperature and thus to process each flat substrate under the same condition.
Further, since the uniformity of the internal temperature of the flat substrate is lowered, it is difficult to uniformly process the entire surface of the flat substrate, and particles are likely to be generated.
Therefore, the inventors have proposed an electrostatic chuck that, even when a flat substrate is repeatedly loaded on the electrostatic chuck and the time required to perform a waferless dry-cleaning process is lengthened, can prevent a large variation in thermal conductivity between the electrostatic chuck and the flat substrate over time, prevent particles from being generated, prevent particles from being adhered to the rear surface of the flat substrate, have a uniform electrostatic attraction force, and prevent the flat substrate from being separated after the application of voltage stops (see Japanese Patent Application No. 2004-223995).
In the electrostatic chuck, a plurality of protrusions are provided on one surface of a body, and a plurality of minute projections are provided on each of the top surfaces of the protrusions. In addition, the top surface of each of the minute projections is referred to as a mounting surface on which the flat substrate is loaded, and the total area of the plurality of minute projections is equal to or larger than 1% and equal to or smaller than 40% of the area of the top surface of the protrusion.
In the electrostatic chuck, the flat substrate is loaded on the mounting surfaces, which are top surfaces, of the minute projections provided on the top surfaces of the protrusions. Therefore, even when the flat substrate is repeatedly loaded on the electrostatic chuck and the time required to perform the waferless dry-cleaning process is lengthened, it is possible to prevent thermal conductivity between the electrostatic chuck and the flat substrate from varying over time.
However, in the electrostatic chuck, it is also preferable to widen a temperature range in which the temperature of the flat substrate can be maintained at a constant value by controlling the pressure of a cooling gas and to reduce a variation in thermal conductivity between the electrostatic chuck and the flat substrate overtime.